1. Field of the Invention
Seismic data processing with particular emphasis on identifying and suppressing unwanted frequency components from seismic data based on the assumption that surface-consistent contributions can be identified in accordance with trace organization.
2. Discussion of Related Art
Although the art of seismic exploration is very well known, it will be briefly reviewed to provide definitions of technical terms to be referenced herein.
An acoustic source of any desired type such as, by way of example but not by way of limitation, a vibrator, an explosive charge, an air or gas gun, or an earth impactor, is triggered to propagate a wavefield radially from the source location. The wavefield insonifies subsurface earth formations whence it is reflected therefrom to return to the surface. The mechanical earth motions due to the reflected wavefield are detected as electrical signals by an array of seismic receivers or receiver groups distributed at preselected spaced-apart group intervals, at or near the surface of the earth, along a designated line of survey.
Hereafter for brevity, the term "receiver", unless otherwise qualified, means either a single seismic receiver or a relatively compact group of interconnected seismic receivers. The mechanical motions detected by the receivers are converted to electrical or optical signals which are transmitted over ethereal, electrical or optical data-transmission links to a multi-channel recording device. Usually, each receiver is coupled to a dedicated recording channel. An array may encompass many tens or hundreds of receivers which are coupled by a transmission link to a corresponding number of data-recording channels. To reduce the need for an excessive number of individual data transmission lines between the receivers and the recording channels, the receivers share a relatively few common transmission lines and the signals from each receiver are multiplexed into the appropriate data-recording channels by any convenient well-known means.
In operation, the selected source type successively occupies a plurality of source locations along the line of survey, emitting a wavefield at each location. After each emission, the source is advanced along the line by a multiple of the receiver spacing interval. At the same time, the receiver array is advanced along the line of survey by a corresponding spacing. In other arrangements such as for use with three-dimensional studies, the sources and receivers are emplaced at the intersections of a uniformly-spaced coordinate grid.
The distance between a source location S.sub.i and the nth receiver location R.sub.j is defined as the offset O.sub.l. The reflected wavefield trajectory impinges on a subsurface formation at the midpoint G.sub.k between the source location S.sub.i and the receiver location R.sub.j. As before stated, the electrical output from the nth receiver is recorded in the nth recording channel C.sub.n.
In its trajectory through the earth, a propagating wavefield is influenced by instrumental characteristics of the source, the receivers and the recording channels. The wavefield is further influenced by the filtering effect of the earth materials through which it passes and by effects due to earth-source-receiver coupling. For purposes of this disclosure, those influences will be referred to collectively as organizational effects.
In addition to organizational effects, the wavefield may become contaminated by noise. By definition, any seismic signal that interferes with the clear reception of desired data from a target formation is noise. A perceived seismic wavefield may be described by an amplitude term and a spectral (frequency) term. Noise may take the form of random anomalous transients that distort the amplitude component, the spectral component or both.
It has been shown by Taner et al. (see Surface Consistent Corrections, Geophysics, v. 46, n. 1, January 1981, pp 17-21) as well as in U.S. Pat. No. 4,866,679, issued Sep. 12, 1989 to Ronald E. Chambers and assigned to the assignee of this invention and incorporated herein by reference, that unwanted amplitude-related noise can be isolated by decomposing seismic reflection-data measurements into organizational components of source, receiver, offset, common mid-point, and channel-consistent quantities as follows: EQU A.sub.ijh =S.sub.i *R.sub.j *G.sub.hk *O.sub.l *C.sub.n *N, (1)
where
A.sub.ijh is the measured amplitude level of the signal at time window h of a wavefield received at receiver R.sub.n at location j, offset from a source S at location i by a distance l after reflection from subsurface midpoint G.sub.h at position k as recorded in channel C.sub.n. N is the noise component.
Equation (1) can be linearized by taking the natural logarithm of both sides and adding, thus: EQU ln A.sub.ijh =ln S.sub.i +ln R.sub.j +ln G.sub.hk +ln O.sub.l +ln C.sub.n +ln N. (2)
The energy and spectral contributions due to the respective components are derived from common source gathers, common receiver gathers, common midpoint gathers, common offset gathers as found from well-known conventional multi-fold coverage methods and as described and illustrated in the Taner reference above cited. Channel defects may be found by simple inspection of the data recordings derived from respective gathers.
Formulation (2) is arranged in a matrix which is inverted using, for example but not for limitation, an iterative Gauss-Seidel equation-solver routine to solve for the coefficients of the component terms and the noise component N as explained in the '679 patent. Re-combining (2) after removal of the noise component N yields a noise-free model trace B.sub.ijh for time window h.
Given a signal having an average measured amplitude level, A.sub.ijh, for an individual trace within time window h, the level of the signal is conditionally scaled to become scaled signal X.sub.ijh as follows: ##EQU1## where m is a user-defined integer equal to or greater than unity and .delta. is some threshold value such as 1.0.
The principles above discussed apply to noise abatement with respect to anomalous signal amplitude levels. Taner also applied the method to anomalous phase shifts due to near-surface irregularities. He did not, however, address himself to spectral interference as such nor did the '679 reference consider the spectral content of undesired transients.
Spurious monochromatic or limited-bandwidth polychromatic frequencies sometimes seriously contaminate seismic recordings. Sources may include 50- or 60-Hz power-line hum, periodic pulsations from water pumps or pipe-line booster pumps, ship-generated propeller noises, sea-creature conversations and water bottom multiples. The interfering spectral noises overlap the seismic signal spectrum and wreak havoc with the desired signals, particularly the very weak relatively low-frequency signals reflected from layers deep within the earth. In some cases, unwanted signals are minimized by spatial or instrumental filtering. Sometimes the undesired spectral components can be removed by bandpass filtering. But often the spectral noise overwhelms the known treatments.
There is a need for a processing method for discriminating against residual anomalous frequency spectra. The method mst be efficient and economical of processing time.